How can I bake textures on uneven goemetry without tearing and stretching
This issue can be solved by adding evenly spaced loops along the long axis of the model and then later deleting them after the normal map is baked.

The problem here is that altering a models geometry after the Tangent map is baked can invalidate a Tangent Space map.  

There is a way around this:  

(i) We can bake to an Object Space map first

(ii) Now the extra loops can be removed

(iii) Finally, a second bake pass that uses the Object Space map as Input for the Material normals can then be used to transfer the Object Space map info to a Tangent Space map and the resulting map will be valid for the low poly model that has had its control loops removed after the bake has taken place.

You can safely treat this second bake pass as a color transfer more than anything else.  Blender internal makes this quick and straight-forward. With practice, it shouldn't take more than 30 seconds even if you are being patient and going slow. Back to this in a minute.  

To start with, add the control loops to one of the boards and then by using linked-duplicates, move and rotate the other boards into place; now most of the setup can be done on only one of the components and the changes will be reflected on all of them.

Here the loops are added in by using Ctrl+r and then pressing the NumPad(+) key about 30 times to increase the number of segments.



Now the pieces can be joined into one Object so the UV's can be layed out(mostly)properly as shown next.  Try to use all the available space so that the texture resolution is not wasted.  Also, try to leave a fair amount of space between the UV Islands and the borders so the textures do not bleed across the seams.  Mipmapping can exaggerate this issue so it's best to not skip this last detail.



Now bake to an Object space map and the results should be error free so long as the 'Selected To Active' settings are correct with regards to Bias/Distance/Cage controls that are available when Using Selected To Active for baking from a High-Poly Model to a Low-Poly model.



There are two good reasons to start with Object Space maps in a situation like this.  First, the models' geometry can be further reduced after the bake has taken place without invalidating the Normal Map because Object Space Maps are not susceptible to that problem.

Second, Object Space maps can be blurred without issue in a 2D Editor such as GIMP. 

Blurring Tangent Space maps will almost certainly cause you problems.

The following animation shows how to quickly remove all the extra control loops prior to converting the Object Space map to Tangent Space.

To start with, Select the end faces and then use Ctrl+i to invert the Selection.  Now Use Ctrl+Numpad(-) to contract the Selection.  

Switch to Edge Selection Mode and hold down the Shift and Alt keys and start mouse-clicking the long edges one at a time.  The combination of keys and clicks will remove the long edges from being selected.

When only the extra loops are still selected, use x+g to delete all the loops. Now there should only be the basic framework of the pieces just as before.



Next add a Blender Render(Blender Internal) Material and a texture to the material which should be set as an Object Space normal map using the settings shown in the following image.  The bake settings are also shown at the top right. 



Now when you press bake, you should end up with a Tangent map that does not require the use of Selected to Active and its encoding should match up with the Low-Poly model.

Now for the Materials texture, replace the Object Space map with the Tangent space map and set the Image Sampling to 'Tangent' instead of 'Object' and you should have the following results.

Here is the Tangent map being used to light the model with GLSL.



Next the same normal map is being used in Cycles.  A Glossy shader is being used to exaggerate the normal map's detail in order to look for errors.  Glossy is basically a worse-case scenario for normal maps.  If looks good for shiny, it should always look good.(maybe not if it's really small and far away; then shiny will cause twinkles when the normal map detail is smaller that the computer screens pixels. :) &lt;-You can fix that by blurring the map by a few pixels in GIMP, Krita, Photoshop, etc.